For some reason, the Viva method just does not work that well for me. But it sounds like maybe your icing is too thick so once it crusts over, it cracks once smoothed. Try thinner icing, see if that works. I had the same thing happen to my on my camo cake and I am sure it is because the icing was way too thick.
I used it this weekend and got cracks also....I am thinking it's because my boards weren't very thick/strong and I got cracks because the cake board would give and move slightly......Honestly, the cracks were not even noticable to the customer....Only I noticed them....I tried gently touching the cracks with my fingers and that patted them down a little.
I'll be anxious to see what kind of responses you get from this post also!
I have this problem too. But I really believe it is from where the cake board gives when trying to pick it up. It does not happen to me when I used boards that are strong and do not give.
As far as repairs, pat it gently back in place over the cracks and see if you are able to seal the cracks up. It may not be perfect but it should not be as noticable.
Good luck!!
I think the others are correct - your icing is too thick, and one other thing I have learned that is if I wait too long to smooth - it will also crack. You need to smooth it pretty quickly IMO - I only wait a few minutes (3 - 5 minutes) then I smooth. It works flawlessly then. (and of course, make sure you have a sturdy enough board under the cake!)
I wouldn't wet your finger....just try using the viva again over the crack. I almost messed up my purse cake (in my photos) with the toothpicks that I used to put the strap into the cake....they poked out of the sides of the already smoothed and crusted side. I used the viva again and IT WORKED!!!
I think you are waiting too long after it crusts. If you wait too long it just crusts over too much and will crack. Just let it sit for 10 or 15 minutes (or however long it takes for your icing to just crust over) and smooth. Dont wait like hours or overnight.
Does it crack and it looks like it is goin to fall off the cake?...I just did one..with the VIVA...It cracked first then bubbled...and fell off the cake..Is that what you meant too???....Is that cause of the VIVA????
Sorry...I am freaking out here...then I read your message...maybe I found my reason....Thanks Nicole
To keep frosting from sugaring add a drop of white distilled vinegar. It will also help keep white frosting white and shiny.
http://www.vinegartips.com/cooking/
HTH
hmm does the vinegar change the flavor or aroma of the icing? Vinegar is good for just about everything. Amazing stuff....I shouldn't be surprised.
It's a little late but I hope it's some helpful info. Attached you'll find the picture of the first and ONLY cake I've had the buttercream crack on. I know for a fact this was due to the cake board. I only used one and it was a half sheet cake.... my fault totally lol. But when I removed the cake from the box at the party, since the board had already bent itself when I picked it up it did the same and I just let it a touch not so much it would ruin but enough that it stuck the icing back together and NO ONE noticed, except me of course lol.
Hope this helps
Notes for the picture: Crack starts in between the P's and goes down in between the H and D and in between the S's lol. But it fixed itself! 
maybe the cake boards were not sturdy enough for support have had that happen to me
wow that's a neat tip! WISH i had it before I had my last cake picked up- I used the crusting buttercream icing and it seemed to get cracks all over! And the cake was on a masonite board so it wasn't the board! It drove me nuts.... I also got a lot of those little "air pockets" or "Holes" in my icing and I don't know why......... I beat it really slowly....and I added powdered sugar a little at a time.... I don't know what went wrong but it had holes all throughout it....
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